Because on Android it's not that easy. Being able to publish a successful paid version requires two things: a working store and customers who are used to purchase applications. On Android neither one of those prerequisites are in great shape. The Android market only works on some countries and even there the store volumes are quite low as people are not used to buy software like on iPhone.

That is why we released the Android version as ad funded. People get the game for free but we can still cover our costs from the ad revenues.

Game business is maybe not all about the money as you need to have a real passion for making games to make them great. But money is still the bottom line because without it no company can function, no matter how much love there is.